In FR (frame relay), data is sent in packets which have a two byte header and have a variable size data section (payload) which ranges in size from two to 8187 bytes. FR is connection oriented, with each packet including a connection identifier, the packets of a given connection constituting a logical frame relay connection. Each FR logical connection is identified by a DLCI (data link connection identifier). Currently, DLCIs range from 0 to 1023 with DLCI-16 through DLCI-1007 allocated for data connections. Typically, DLCI-0 is reserved to function as a signalling connection. This connection is used to set up new logical connections for example.
Between two FR switching elements there is a capability of assigning a transfer priority to each connection typically in the range of 0 to 15. FR messages include C-plane messages which are used for signalling, and U-plane messages which are used for data, the C-plane messages requiring a higher priority than U-plane messages. Between two FR switching elements high priority LMI (local management interface) C-plane messages are used to implement a handshaking protocol. This high priority assigned to LMI traffic thereby ensures that essential signalling information can get through the network notwithstanding congestion which may otherwise exist.
In ATM, data is sent in cells which have a fixed size 53 bytes including a five byte header. ATM is also connection oriented with ATM cells being carried in VCCs (virtual channel connections). One VCC represents one connection in much the same way that one DLCI represents one FR connection. Each VCC has a QoS (quality of service). The QoS's presently defined include CBR (constant bit rate) typically used for voice, VBR-rt (variable bit rate--real-time) used for real-time sensitive services, VBR--non-real-time used for services which are not real-time sensitive, UBR (unspecified bit rate) used for low priority traffic, and ABR (available bit rate) which has recently been introduced. The QoS for a VCC is fixed across all DLCIs carried over the VCC.
There needs to be the capability to connect FR devices to ATM devices and vice versa, and this capability is usually implemented by some kind of IWF (interworking function). At an IWF, this has consisted of some sort of mapping between VCCs, and incoming/outgoing logical frame relay connections (DLCIs).
Two existing FR-ATM interworking solutions adhere to two schemes of multiplexing presented in ITU-T recommendation I.555. There is also a solution proposed by FRF.5 which builds upon that of ITU-T I.555. In all of these solutions, control/signalling information is carried with no distinction from data across each ATM connection. In other words, the ATM network gives the same priority to signalling messages as to the data itself.
Historically, FR has been thought of as being applicable to non-real time applications, and as being most suitably carried by ATM VBR-nrt. Since signalling packets can be expected to satisfy VBR-nrt behaviour and the data can also be modelled as VBR-nrt, the above problem of giving the same priority to signalling as data has not been significant. However, now real-time applications for FR exist, for example voice over FR, in which case it would be useful to be able to carry FR traffic with different QoS and still be able to reliably transmit control/signalling information. The existing solutions do not permit this. If for example, the ATM UBR service is used for the data and control it is likely that some packets will be lost. While this may not be serious if a regular data packet gets lost, it is a problem if control and signalling information is not getting through reliably.